Note: Javascript is disabled or is not supported by your browser. For this reason, some items on this page will be unavailable. For more information about this message, please visit this page: About CDC.gov.

3.4 Prevalence of Soil-Pica

Historical Document

This Web site is provided by the Agency for Toxic Substances and Disease Registry (ATSDR)
ONLY as an historical reference for the public health community. It is no longer being maintained and the data
it contains may no longer be current and/or accurate.

The second was: "What is the prevalence
rate of soil-pica behavior among children, especially preschool children? among
adults? among pregnant women?" The
panelists stressed that the prevalence of pica behavior is highly dependent
upon how it is defined, and eventually defined the various terms (e.g., soil
ingestion, soil-pica, geophagy) used throughout the workshop. Even with a clear
definition of soil-pica, however, the panelists had difficulty quantifying the
prevalence among subpopulations, given the lack of extensive soil ingestion
studies. Nonetheless, the panelists agreed that soil-pica exists and that ATSDR
should continue to evaluate the public health implications of all types of soil
ingestion behavior, including soil-pica. Following is a summary of the discussion
that led to this conclusion.

The importance of clearly
defined terminology. The panelists repeatedly stressed that the prevalence
of soil-pica behavior depends on how one defines this behavior. As an example
of their concern, panelists noted that they have seen "soil-pica" defined
in terms of quantities of soil ingested and whether the ingestion behavior
is abnormal, intentional, or repetitive (BL, JM, DV). One panelist cautioned
against limiting pica behavior to abnormal soil ingestion levels, because
such a restricted definition would overlook the fact that children's typical
behaviors can lead to relatively high soil/dust ingestion rates (see Section
3.1) (NF). The other panelists stressed that a clear, unambiguous definition
of soil-pica must be crafted so that ATSDR can quantify the prevalence using
the various methods discussed in Section 3.3. Given the importance of communicating
with consistent terminology, the panelists defined soil ingestion, soil-pica,
and geophagy at the close of the meeting. Refer to Section 2.0 for these definitions.

Comments on the distribution
of soil ingestion rates. When discussing the prevalence of soil-pica,
two panelists suggested that ATSDR view soil ingestion rates as a continuum,
possibly by characterizing the distribution of these rates (SD, JM). Knowing
the distribution of soil ingestion rates, according to one panelist, would
allow researchers to quantify the distribution of exposures to soil contaminants
at sites where the nature and extent of soil contamination has been determined,
but this panelist was not convinced that the data currently available are
sufficient for estimating this distribution (SD). Another panelist then asked
whether a table from a publication (Calabrese and Stanek, 1998) provides a
reasonable estimate of the distribution of soil intakes (DM). The panelist
responded that he was not sure, because he did not know how the estimate was
derived (SD). (2)
Given the importance of knowing the distribution of soil ingestion rates for
various age groups, one panelist suggested that ATSDR try to estimate these
distributions from the various soil ingestion studies that have been published.

Comments on data reported
in the scientific literature. Though not commenting specifically on the
prevalence of soil-pica behavior, several panelists noted relevant data documented
in the scientific literature. For instance, one panelist noted that a team
of researchers has estimated that 33% of children ingest more than 10 grams
of soil 1 or 2 days a year (Calabrese and Stanek, 1998) (DM). Another panelist
cautioned, however, that this estimate is based on an extrapolation of a short-term
study and not on a study of soil ingestion over an entire year (NF). Another
panelist noted that his analytical studies as well as those published by Ed
Calabrese and Ed Stanek, and by Michael Wong present estimatesAppendix B:
Charge to the Panelists of soil ingestion rates, though they do not have consistent
findings (SD). Another panelist indicated that his research has found that
30% of children (aged 1-3 years) in Rochester, New York, ingest soils, based
on a survey of parents, and that this behavior is associated with a 14% increase
in blood lead levels (BL). This panelist cautioned about assuming that these
findings might apply to other sites and other contaminants. Overall, the panelists
thought their comments confirm that soil-pica exists, but they refrained from
providing quantitative estimates of the prevalence of soil-pica behavior,
largely because the available studies are
limited in duration and not based on a population that represents all groups
of children.

Prevalence of soil-pica among
pregnant women. One panelist noted that women in urban areas would likely
not dig and process their own geophagical clays, but would likely purchase
them or obtain them from areas where they were reared when relatives came
to visit (DV). Further, he doubted that pregnant women in urban areas would
consume surface soils from their backyards. Consistent with these comments,
another panelist provided an anecdotal account of stores in the Atlanta area
that sell geophagical clays, which pregnant women might consume (JM). The
panelists eventually agreed that studies have not been conducted to determine
the extent to which pregnant women exhibit soil-pica behavior, though they
suspected that consumption of residential soils is likely rare.

Variations in soil ingestion
rates with age. Citing his own research, one panelist indicated that
the percentage of children in his study who ingested soils, as reported by
their parents, was 3% for 6-month-old children, 30% for 12-month-old children,
31% for 18-month-old children, and gradually lower percentages for older children
(BL). (Note: this research
did not ask about "pica behavior," but rather asked parents whether their
children ingest soils.) Somewhat consistent with this finding, another panelist
added that he believes intentional soil ingestion behaviors decrease as children
reach roughly the age of 3 (DV). He attributed this apparent decrease to the
observation that parents try to control certain behaviors (including soil
ingestion) as children reach ages when they can reason, while they overlook
these behaviors when children are younger. One panelist indicated that the
available data on this topic are extremely limited (NF).

Implications of the prevalence
of soil-pica behavior.
The panelists offered various opinions on the implications of soil-pica behavior,
regardless of not knowing the exact extent to which it occurs. One panelist,
for example, believed that a significant number of children exhibit soil-pica
behavior, but added that an insignificant number of children might develop
adverse health effects (NF). She noted that ATSDR ultimately needs to consider
many factors other than the prevalence of soil-pica behavior (e.g., the nature
and extent of contamination and bioavailability) to put the health concerns
into perspective. This panelist added, however, that ATSDR should err on the
side of possibly overestimating the prevalence of soil-pica behavior, given
that it might be associated with adverse health effects. Another panelist
agreed, noting that his research has found soil ingestion, as reported by
parents, to be a significant risk factor for childhood lead poisoning (BL).
The panelists agreed that ATSDR should continue to evaluate the public health
implications of soil-pica behavior, despite the uncertainties associated with
the nature and extent of soil-pica. Most panelists agreed that ATSDR should
try to validate the public health significance of soil-pica behavior through
site-specific studies.

2. At this point, an observer clarified that the author of the publication apparently extrapolated the
results of a 2-week analytical study to soil ingestion rates over the course of the year. This extrapolation
reportedly assumed that the variability of soil ingestion rates over a year is greater than that which was
observed in 2 weeks--an assumption the observer questioned given that the study of concern was conducted
during the summer, when soil ingestion rates would likely be greatest and perhaps most variable.